1. Field of the Invention
The present invention relates to a powder-image developing device utilized in an image forming apparatus capable of performing an electrophotographic copying process, such as, for example, a copying machine, a printer or the like.
2. Description of the Prior Art
Hitherto, a developing device such as shown in FIGS. 6 and 7 has been proposed. Referring to FIGS. 6 and 7 which are a side sectional view of the developing device taken in a direction perpendicular to the axis of rotation of a photoreceptor drum and a perspective view of a portion of the developing device, respectively, the illustrated developing device 20 comprises a generally box-like casing 21 having defined therein a sleeve chamber in which a developing sleeve 22 is rotatably supported with a portion of the outer periphery thereof exposed to and confronting the photoreceptor drum 100 on which a latent image is formed. A magnetic roller 23 is coaxial to and fixedly disposed inside the developing sleeve 22 and comprises a plurality of magnetic pole members S1 to S3 and N1 to N3 disposed circumferentially thereon in an alternating fashion as shown.
The casing 21 also has upper and lower chambers 24 and 25 defined therein one above the other on one side of the developing sleeve 22 remote from the photoreceptor drum 100. The upper and lower chambers 24 and 25 are substantially partitioned by a partition wall 26 having opposite ends connected, or otherwise formed integrally, with opposite end walls of the casing 21 so as to extend in a direction generally parallel to the longitudinal axis of the developing sleeve 22. The partition wall 26 has a width so selected as to define elongated openings 27 and 28 on respective sides thereof, though which the upper and lower chambers 24 and 25 communicate with each other.
As best shown in FIG. 7, screw feeders 29 and 30 are rotatably supported within the upper and lower chambers 24 and 25. The screw feeder 29 is driven in a direction opposite to the direction of rotation of the screw feeder 30 so that a developing material conveyed by the screw feeder 29 in one direction Y1 within the upper chamber 24 can be conveyed by the screw feeder 30 in the opposite direction Y2 within the lower chamber 25. The developing material utilized in the illustrated developing device comprises a mass of toner material and a mass of carrier material and after being supplied into the lower chamber 25, cascades under the force of the screw feeder 30 onto the outer peripheral surface of the developing sleeve 22.
The developing material supplied onto the developing sleeve 22 is retained in the form of, a magnetic brush of bristles on the outer peripheral surface of the developing sleeve due to the magnetic forces that are exerted thereon by the magnetic roller 23. During the rotation of the developing sleeve 22, the magnetic brush of bristles is transported in a direction, shown by the arrow a in FIG. 6, towards a developing station S where a minimum space is established between the developing sleeve 22 and the photoreceptor drum 100. During the transportation, the magnetic brush of bristles passes through the front opening 17, then through a spacing 27, delimited between a front wall of the casing 21 and the outer peripheral surface of the developing sleeve 22, and finally through a regulating gap delimited similarly between the developing sleeve 22 and a bristle height regulating plate 31 for regulating the height of the magnetic brush of bristles thereby adjusting the amount of developing material supplied towards the developing station S. A portion of the developing material forming the magnetic brush of bristles which has been squeezed by the regulating plate 31 and is not therefore transported towards the developing station S flows backwards in a direction counter to the direction a of rotation of the developing sleeve 22, returning through the spacing 27 into the upper chamber 24 for mixing with the developing material within the upper chamber 24.
The developing material within the upper chamber 24 is, during the rotation of the screw feeder 29, conveyed in the direction Y1 while being successively forced to flow into the lower chamber 25 through the rear opening 28 remote from the developing sleeve 22.
The prior art developing device having the structure described with reference to FIGS. 6 and 7, has some problems inherent therein. Specifically, that portion of the developing material, carried by the developing sleeve 22 to the developing station S, which has been left unused in the formation of a powder image on the photoreceptor drum 100 is not removed from the developing sleeve 2 even after it has been moved past the developing station S during the continued rotation of the developing sleeve 22. During the next succeeding cycle of rotation of the developing sleeve 22, the developing material is supplied onto the developing sleeve 22 while the latter still retains that portion of the developing material which was not used during the previous cycle of rotation of the developing sleeve 22 and was left unremoved therefrom.
Because of this, stratification of the developing material magnetically attracted onto the outer peripheral surface of the developing sleeve 22 has been found to occur. Considering that the magnetic brush of bristles is equivalent to a layer of developing material on the outer peripheral surface of the developing sleeve 22, and, also considering that that portion of the developing material having moved past the developing station S without being completely used for the formation of the powder image on the photoreceptor drum 100 contains toner particles of a density reduced as compared with that of the toner particles in the developing material supplied to the developing station S, the supply of the developing material for use in the next succeeding cycle of rotation of the developing sleeve 22 results in a layer of developing material containing a lower density of toner particles being formed at a lower region thereof adjacent the outer peripheral surface of the developing sleeve 22 and a higher density of toner particles being formed at an upper region thereof remote from the outer peripheral surface of the developing sleeve 22.
Moreover, most of the concentrated toner particles in the upper region of the layer of developing material are, during the passage of the magnetic brush of bristles underneath the regulating plate 31 and, hence, through the regulating gap, squeezed from the lower region of the same layer of developing material. Therefore, the developing material containing a reduced density of the toner particles tends to be supplied towards the developing station S. This is problematic in that, when a number of copies are repeatedly made from an identical original, the image density is abruptly reduced with the increase in the number of the copies, constituting a cause of a reduction in image quality.
Also, in the prior art developing device, a number of similar recirculating paths 32 are formed around the partition wall 26 and over the lengthwise direction of any one of the screw feeders 29 and 30 for facilitating the recirculation of the developing material from the upper chamber 24 back to the upper chamber 24 first through the rear opening 28, then around the screw feeder 30 in the lower chamber 25 and finally through the front opening 17 as indicated by the arrow-headed line in FIGS. 6 and 7. The developing material within each of the upper and lower chambers 24 and 25 is of course conveyed a small distance by the associated screw feeder 29 or 30 in the associated direction Y1 or Y2 and is first forced by the screw feeder 29 to fall into the lower chamber 25 through the rear opening 28. Then the material is readily scooped up by the screw feeder 30 so as to enter the upper chamber 24 through the front opening 17.
The presence of the number of the recirculation paths 32 has been found to adversely affect the fluidity and the dispersibility of the developing material. When an original having a solid area at a location presumably corresponding to one of the opposite ends of the developing sleeve 22 is copied on a copying paper, the solid area will be reproduced on the copying paper in a reduced image density and the subsequent copying of character images results in the lowering of the character reproducibility.
When a toner sensor for detecting the concentration of the toner component in the developing material is disposed in the vicinity of one of the opposite ends, and when a mass of toner particles is replenished, the toner sensor will require a relatively long time before it detects the concentration of the toner particles then replenished. Therefore, it often occurs that an operator may excessively replenish the toner particles, resulting in the creation of an over toner condition which may bring about the formation of fog and/or toner dusting. Once this occurs not only is the copy quality reduced, but the interior of the machine is also contaminated.
Conversely, even when an area where the concentration of the toner particles is reduced is formed, the reduction in toner concentration cannot be quickly detected, and the inflow of the toner particles into that area where the toner concentration is reduced tends to be delayed, resulting in the enhancement of the reduced image quality. Yet, the insufficient fluidity of the toner particles often constitutes a cause of insufficient charging of the toner particles.